<!doctype html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>js示例4-10-17</title>
	<script>
		var stringA = "This is a student.";
		var stringB = stringA.substr(5,2);
		alert(stringB);
		
	</script>
</head>
<body>
	
</body>
</html>